1. Field of the Invention
The present disclosure generally relates to formulations for the treatment of substrates. More particularly, the disclosure relates to size press formulations and methods of treating paper using the size press formulations.
2. Description of the Related Art
A paper mat typically includes water and solids. The solid portion includes fibers (typically cellulose-based fibers) and can also include filler. Increasing the strength of the paper mat would allow one to increase the proportion of solids that is filler content. This would be desirable because it reduces raw materials costs, reduces energy needed in the papermaking process, and increases the optical properties of the paper.
Fillers are mineral particles that are added to a paper mat during the papermaking process to enhance the resulting opacity and light reflecting properties of the paper. Fillers are mostly inorganic particles or pigments used to increase the opacity or brightness, reduce the porosity, and/or reduce the cost of the paper or paperboard. Some examples of fillers include kaolin clay, talc, titanium dioxide, alumina trihydrate, barium sulfate, magnesium hydroxide, pigments such as calcium carbonate, and the like.
Calcium carbonate filler comes in two forms, ground calcium carbonate (GCC) and precipitated calcium carbonate (PCC). GCC is naturally occurring calcium carbonate rock and PCC is synthetically produced calcium carbonate. Because it has a greater specific surface area, PCC has greater light scattering abilities and provides better optical properties to the resulting paper. For the same reason, however, PCC filled paper mat produces paper that is weaker than GCC filled paper.
Paper strength is a function of the number and the strength of the bonds formed between interweaved fibers of the paper mat. Filler particles with greater surface area are more likely to become engaged to those fibers and interfere with the number and strength of those bonds. Due to its greater surface area, PCC filler interferes with those bonds more than GCC. As a result, papermakers are forced to make an undesirable tradeoff. They must either choose to select a paper with superior strength but inferior optical properties or they must select a paper with superior optical properties but inferior strength.
Increasing filler loadings, such as PCC, while maintaining basis weight in an uncoated free sheet is desirable due to gains in optical properties and the cost difference between filler particles and fibers. However, as mentioned above, papermakers are limited in the amount of fillers in the final product due mostly to a net loss in strength. Tensile strength, z-directional tensile strength, and the tendency of the paper to shed filler particles (i.e. dusting) during typical handling processes, such as printing, are some of the main properties affected.